Heretofore, an active-matrix (hereinafter, referred to as AM) type organic EL display has been studied as a light emitting display element provided with a pixel comprising an organic EL element and a driving circuit in a matrix pattern.
FIG. 23 illustrates a structure of the pixel, and FIG. 24 illustrates a structure of the AM type organic EL display. In the AM type organic EL display, a gray-scale (gradation) control is performed by controlling light emission intensity.
It is pointed out that a voltage-light emission characteristic of the organic EL element changes with time and the characteristic of a thin film-transistor (hereinafter, referred to as TFT) used in the driving circuit varies.
Hence, to realize a display without variation, it is necessary to use a driving circuit and driving method which are hardly affected by time aging of the organic EL element characteristic and the characteristic variation of the TFT.
As a first technique to solve this problem, a driving circuit as illustrated in FIG. 25 has been proposed (for example, Japanese Patent Application Laid-Open No. 2002-517806; corresponding to International Publication WO99/65011).
While the characteristic variation of the TFT can be regarded as variation between the threshold value and mobility, in the first technique, a current is supplied from the outside to the TFT (Tr1) which is short-circuited between a gate and a drain inside the driving circuit. By so doing, the gate of the TFT can be set to the voltage in which the current is let flow from the outside according to the threshold value and mobility of the TFT.
After that, when a current channel is directed to an organic EL element LED1 after the gate voltage is held, the voltage between the gate and the source of the TFT becomes the same as the voltage in which the current is let flow from the outside. Hence, the TFT acts as a current source for supplying a constant current of the same magnitude as the current from the outside, and can let flow a current of the same magnitude as the current from the outside to the organic EL element.
Consequently, in the first technique, if there is no variation in the current from the outside, regardless of the characteristic variation of the TFT, a constant current can be supplied to the organic EL element, so that a uniform display can be performed.
Further, as a second technique to solve the above described problem, a driving circuit illustrated in FIG. 26 has been proposed (For example, Japanese Patent Application Laid-Open No. 2003-223136). The second technique is provided with a voltage comparator (CMP) and a switch TFT (Tr1) to which output of the voltage comparator is connected to the gate and the source and drain are connected to the power source and the organic EL element LED1.
In the second technique, after a reference analogue voltage is held in the input of the voltage comparator, sweep voltages of a suitable range are applied in order. The voltage comparator outputs a voltage of high level (low level) if the sweep voltage increases (decreases) as compared with the reference voltage, and can control ON/OFF of the application of the voltage to the organic EL element since the Switch TFT turns ON/OFF.
Consequently, the light emission time of the organic EL element can be controlled by an applied waveform of the sweep voltage, and a multi-gray-scale display can be performed.